1. Field of the Invention
The present invention relates to a friction material for brakes, clutches, etc. used in railroad cars, trucks, passenger cars, etc., especially a friction material that engages with a rotor or drum comprising an aluminum alloy reinforced by a hard material.
2. Related Background Arts
FC 20 to 30 cast iron traditionally has been mainly used as a material for rotors and drums used in brakes, clutches, etc. However, in passenger cars, for example, an aluminum alloy reinforced by a hard material has increasingly been used to reduce the weight of the members to reduce fuel consumption.
Not only are rotors and drums required to be light weight but also to have high resistance to heat and wear. To meet these requirements, they are produced, for example, by the following two methods:
(a) a casting method in which a molten aluminum alloy is mixed with a hard material composed of a ceramic, such as silicon carbide (SiC), alumina (Al2O3), silicon oxide (SiO2), zirconium oxide (ZrO2), and magnesium oxide (MgO); and
(b) a hot-press forming method in which a sheet of aluminum foil and a sheet of pre-impregnated carbon fiber are alternately laminated.
Because the method (b) is complicated in manufacturing process and therefore costly, the method (a) has been adopted mainly in recent years.
Friction materials are shifting to the use of a non-asbestos family, in which no asbestos is used, in consideration of human health. Non-asbestos-family friction materials use a heat-resistant organic fiber, a glass fiber, a metal fiber, etc. for the fibrous base material; phenolic resin for the binder; graphite for the friction adjusting agent; and barium sulfate, calcium carbonate, etc. for the filler.
When applying the brake, the friction between the friction material and the rotor or drum generates heat. Therefore, friction materials are required to have a stable and high coefficient of friction at the rubbing surface, which engages with the rotor or drum, in a wide range of temperatures from below-freezing point to hundreds of degrees. They are also required to have sufficient wear resistance.
The published Japanese patent application Tokukaihei 6-228539 discloses a combination of a non-asbestos-family friction material and a rotor composed of an aluminum alloy reinforced by a hard material. This patent application explains that the hard materials to be added to aluminum-alloy rotors generally have a Mohs hardness of 6 or more in many cases and that the hard inorganic materials to be added to friction materials also have a Mohs hardness of 6 or more, preferably eight or more.
According to this patent application, it is desirable that the hard inorganic material to be added to a friction material have an amount of 0.1 to 30 vol % and have a particle diameter of 0.2 to 250 xcexcm in the case of powder and a diameter of 0.1 to 10 xcexcm and a length of 1 xcexcm to 5 mm in the case of fiber.
The patent application illustrates that examples of the hard inorganic materials include metal carbides such as silicon carbide (SiC); ceramic materials composed of alumina (Al2O), silicon oxide (SiO2), zirconium oxide (ZrO2), magnesium oxide (MgO), or a combination of these; and hard metals such as various intermetallic compounds and a nickel-chrome alloy. It also explains that the conventional art using a combination of such a rotor and friction material exhibits not only excellent braking force and wear resistance but also offers a superior effect on the reduction of the tendency to attack the mating surface, that is, the rotor surface that engages with the friction material.
Another Japanese laid-open patent application, Tokuhyouhei 8-510003, states that when a porous copper powder is added to a non-asbestos friction material, which is usually used as the friction material to be coupled with an aluminum-alloy rotor, a xe2x80x9csurface glazexe2x80x9d is formed on the surface of the rotor, thereby stabilizing the friction property. According to the patent application, the xe2x80x9csurface glazexe2x80x9d is a coating consisting mainly of organic constituents, and the formation of the coating is promoted by the porous copper powder.
As the rubbing between the rotor and the friction material progresses, all the fine powders released from the worn-out friction material are deposited in the form of a coating on the surface of the rotor by the action of the braking heat. The coating is ground by the hard inorganic material in the friction material, leaving exposed portions of the aluminum material on the surface of the rotor. The friction material usually includes a metal fiber that has good conformability with aluminum (a brass fiber in the case of the above conventional art). The metal fiber comes in to contact with the exposed portion of the aluminum material in the rotor and produces a local temperature rise (a heat spot) by the frictional heat.
The heat spot softens the rotor locally, and the aluminum alloy and the metal fiber in the friction material seize up mutually, generating a ring-shaped large groove referred to as xe2x80x9ca scoringxe2x80x9d on the friction surface of the rotor. In the present invention, the term xe2x80x9ca scoringxe2x80x9d is used to mean the foregoing ring-shaped large groove generated on the friction surface of the rotor. The scoring causes not only abnormal wear of the rotor but also of the friction material itself, entailing the generation of braking vibration and a reduction in the coefficient of friction.
There is one more point to mention. As described above, Tokuhyouhei 8-510003 states the formation of a coating called a surface glaze on the surface of the rotor. This coating causes a reduction in frictional force. Therefore, it is necessary to prevent the excessive growth of this coating.
An object of the present invention is to offer a friction material to be coupled with a rotor or a drum consisting mainly of an aluminum alloy reinforced by a hard material. The friction material comprises (a) a base material composed of an organic fiber, (b) an inorganic fiber other than a metal fiber, and (c) a metal powder having a particle diameter of 1 to 180 xcexcm. As mentioned above, a metal fiber is not to be contained.
The inorganic fiber to be used plays the role of suppressing the growth of the coating formed on the rotor. Therefore, the inorganic fiber should not be softened by the frictional heat. It is desirable that the inorganic fiber have a softening point of 850xc2x0 C. or higher.
The frictional heat does not necessarily raise the surface temperature of the friction material uniformly. It may produce a local high-temperature heat spot. Consequently, the friction material is required to have durability at considerably higher temperatures than the melting point of the mating aluminum alloy.
If the inorganic fiber is excessively hard, it grinds the surface of the rotor. If excessively soft, it cannot remove the coating on the surface of the rotor. The inorganic fiber preferably has a Mohs hardness of 3 to 5. It is recommendable to use a potassium titanate fiber or slag wool as the inorganic fiber that has a Mohs hardness of 3 to 5 and a softening point of 850xc2x0 C. or higher.
It is desirable that the foregoing metal powder have a particle diameter of 1 to 180 xcexcm, more desirably 20 to 45 xcexcm. If less than 1 xcexcm, the material-manufacturing process becomes complicated and high-cost. If more than 180 xcexcm, the metal powder may cause an undesirable phenomenon similar to that which occurs when a metal fiber is added as described below.
The particle diameter of 20 xcexcm or more can secure the heat-dissipating quality of the friction material. The particle diameter of 45 xcexcm or less can suppress the mating-surface attacking. Furthermore, the particle diameter of 20 to 45 xcexcm is superior in mixing workability and material cost.
It is desirable that the foregoing metal powder comprises at least one member selected from the group consisting of a copper powder, an iron powder, and an aluminum powder; or comprises at least one member selected from the group consisting of a copper-alloy powder, an iron-alloy powder, and an aluminum-alloy powder. If the metal powder is excessively hard, it may attack the mating material. Therefore, the metal powder is required to be soft to a certain extent and to have high thermal conductivity in order to disperse the frictional heat before prior to the formation of a heat spot.
The amount of the metal powder to be added is determined by the amount necessary to dissipate the heat generated at the time of brake application. It is desirable that the amount be 1 to 20 vol % of the material of the friction material.
The friction material comprises as a whole:
(a) a 5 to 20 vol % organic-fiber base material comprising an aramid fiber;
(b) a 13 to 25 vol % binder resin;
(c) a 5 to 25 vol % inorganic fiber;
(d) a 1 to 20 vol % metal powder having a particle diameter of 1 to 180 xcexcm;
(e) a 10 to 40 vol % friction adjusting agent comprising graphite, molybdenum disulfide, and cashew dust;
(f) a 2 to 30 vol % filler comprising barium sulfate, calcium carbonate, and calcium hydride; and
(g) a 2 to 30 vol % inorganic powder comprising zirconium silicate and mica.
The amounts of these materials are adjusted according to the use and required performance of the friction material.
As described above, conventional friction materials are required to improve their performance when they are intended for use in brakes incorporating disk rotors or drums composed of an aluminum alloy. The present invention offers solutions to meet this requirement. Specifically, scoring that occurs when a composition including a metal fiber is used can be prevented by using the metal powder and inorganic fiber of the present invention in place of the metal fiber. The reduction in the coefficient of friction after a thermal histeresis can be suppressed by the addition of the inorganic fiber of the present invention. If the metal powder has an excessively large particle diameter, it loses its effect. Consequently, it is preferable to use a metal powder having a particle diameter of 1 to 180 xcexcm, more preferably 20 to 45 xcexcm. If the inorganic fiber is excessively hard, it attacks the mating material such as a rotor to a large extent. Therefore, it is preferable to use an inorganic fiber having a Mohs hardness of 3 to 5. Such a selection of material makes it possible to offer a friction material suitable for mating with a disk rotor or drum made of an aluminum alloy.